The invention is applicable for use e.g. in mammography and especially in dental radiography, in which it is possible to distinguish e.g. so-called intraoral imaging, which typically means imaging individual or a few teeth, so-called panoramic imaging, which means producing a layer image, i.e. a tomogram of the dental arch, and so-called cephalometric imaging, which typically means producing a transillumination image of the entire skull area. Often the same dental imaging apparatus is used for taking both panoramic and cephalometric images, and today increasingly also for producing various special images of different parts of the dental arch.
In dental panoramic imaging and cephalometric imaging of the skull area as mentioned above, for example, it is necessary to provide for different purposes apertures of various shapes and sizes between the radiation source and the film or equivalent, in order to be able limit the x-ray beam to apply it to the object to be radiographed according to the imaging requirements in each case. For producing different ray beams, solutions are known whereby the beam is limited by means of various blades, i.e. collimators. Typically, one or more blades having a number of different apertures or shapes for forming different apertures are used. Generally, one aperture is used at a tine, and when the imaging apparatus is to be used in a different mode, the ray beam is limited correspondingly by using a different aperture. However, this type of arrangements have the drawback that the user can only apply those forms of ray beam that are provided in the blades. On the other hand, the space available in the imaging apparatus imposes certain restrictions as to the number of different blades, and the arrangements needed to allow replacement of blades also involves certain problems regarding the convenience of use of the apparatus and the need to store the blades.
Another prior-art solution is to use e.g. a so-called four-blade-type collimator for limiting the ray beam. In such a collimator solution, each one of the four blades is driven by a separate actuator, so this type of solutions are implemented using e.g. four different actuators to move two of the blade vertically and two horizontally, thus allowing a desired aperture size to be provided for the ray beam. A drawback with this type of solutions is the large number of actuators, which also leads to complex electronics for the control of the actuators. Moreover, having a large number of actuators increases the cost of the construction. A further problem is that a relatively large space is needed for the actuators.
A prior-art blade mechanism is disclosed in Finnish patent application no. 894310, corresponding Japanese utility model no. 63-119939. This mechanism comprises two blade elements, which are provided with holes of different sizes and can be driven individually relative to each other in the horizontal direction. The limiting of the ray beam is controlled via interlaced disposition of the holes, and one of the blade elements is provided with a wedge-shaped soft tissue filter for use in certain imaging modes. One of the drawbacks of this solution is that, though the ray beam can be limited in a stepless manner in the horizontal direction, stepless limiting is not possible in the vertical direction since limiting of the beam is implemented using apertures that are provided with notches to allow an appropriate aperture height to be achieved. This solution thus provides only a certain preselected number of aperture heights. A further drawback is an expensive and complicated structure.